1. Field of the Invention
This invention relates to an image heating apparatus used in an image forming apparatus such as a laser printer or a facsimile apparatus using the electrophotographic process to fix an unfixed toner image on a recording medium such as a sheet by heat and pressure.
2. Related Background Art
An image forming apparatus using electrophotography according to the prior art is constructed as shown, for example, in FIG. 5 of the accompanying drawings. In FIG. 5, the reference numeral 201 designates a photosensitive drum, the reference numeral 202 denotes a charging roller, the reference numeral 203 designates a laser exposing apparatus, the reference numeral 204 denotes a reflecting mirror, the reference numeral 205 designates a developing sleeve, the reference numeral 206 denotes a toner, the reference numeral 207 designates a toner container, the reference numeral 208 denotes a transferring roller, the letter P designates a sheet as a recording medium, the reference numeral 210 denotes a cleaning blade, the reference numeral 211 designates a waste toner container, the reference numeral 212 denotes a fixing device, the reference numeral 213 designates a paper cassette, the reference numeral 214 denotes a sheet feeding roller, the reference numeral 215 designates a separating pad, and the reference numeral 216 denotes a high voltage source.
The epitome of the operation of the image forming apparatus will now be described. The photosensitive drum 201 is rotated in the direction of arrow, and is uniformly charged by the charging roller 202 supplied with electric power from the high voltage source 216. A laser beam emitted from the laser exposing apparatus 203 is reflected by the reflecting mirror 204, and thereafter is applied to the photosensitive drum 201, whereby an electrostatic latent image is formed on the photosensitive drum 201. The toner container 207 is filled with the toner 206, and with the rotation of the developing sleeve 205, a suitable amount of toner is subjected to moderate charging, and thereafter is supplied onto the photosensitive drum 201.
The toner 206 on the developing sleeve 205 adheres to the electrostatic latent image on the photosensitive drum 201, and the latent image is developed and visualized as a toner image. The sheet feeding roller 214 feeds the sheets P one by one from the paper cassette 213 in timed relationship with the formation of the toner image.
The separating pad 215 is disposed in abutting relationship with the sheet feeding roller 214, and the coefficient of friction, grounding angle and shape of the surface thereof are adjusted so as to feed only a recording medium during each sheet feeding time. The visualized toner image on the photosensitive drum 201 is transferred onto the sheet P by the transferring roller 208. Any untransferred toner not transferred but residual on the photosensitive drum 201 is collected into the waste toner container 211 by the cleaning blade 210, and the photosensitive drum 201 having had its surface cleaned enters the next image forming process.
Also, the sheet P now bearing the toner image thereon is heated and pressurized by the fixing device 212, whereby the toner image is permanently fixed on the sheet P.
The epitome of the fixing device 212 will now be described. A lengthwise schematic view of the fixing device 212 is shown in FIG. 6 of the accompanying drawings, and a cross-sectional view thereof taken along the line 7xe2x80x947 of FIG. 6 is shown in FIG. 7 of the accompanying drawings. FIG. 8 is a lengthwise schematic view of a temperature detecting portion.
The fixing device 212, as shown in Japanese Patent Application Laid-Open No. 63-31382, uses a film heating process in which a pattern of a resistance heat generating member is provided on a ceramic substrate to thereby form a heat generating member and the heat generating member is used as a heater, which is caused to generate heat to thereby heat a sheet bearing an unfixed toner image thereon through thin film.
The reference numeral 108 designates a heater having a resistance heat generating member 108a formed on a ceramic substrate, and the resistance heat generating member 108a is coated with a glass layer 108b as a protective layer. The resistance heat generating member 108a is supplied with electric power by a power source, not shown, and generates heat. Temperature detecting means 117 abuts against the back of the heater 108 and detects the temperature of the heater 108. The temperature detecting means 117 is comprised of a temperature detecting element (ex. thermistor) 101, a heat-resistant elastic member 102 for elastically holding and pressuring the temperature detecting element 101, a frame 104 supporting the temperature detecting element 101 and the elastic member 102 and having a positioning shape for a heater holder 109, a metal 105 molded integrally with the frame 104 and electrically connected to the temperature detecting element 101, and a heat-resistant protective sheet 103 (film) for covering the temperature detecting element 101 and the elastic member 102 and positioned by the metal 105.
The temperature detecting element 101 is vertically movable by an amount corresponding to the expansion and contraction of the elastic member 102 with the aid of a groove formed in the frame 104. The temperature detecting means 117 is positioned by the heater holder 109 and is biased toward the heater 108 by a pair of springs 106. The reference numeral 107 denotes a spring supporting member. The amount of electric power supplied to the heater is controlled by a CPU, not shown, so that the detected temperature by the thermistor may become constant.
The heater holder 109 supports the heater 108 and is molded of heat-resistant resin such as PPS or liquid crystal polymer and serves also as a guide member for expediting the smooth rotation of fixing film 111.
A heater clip 114 and a heater connector 112 for supplying electricity to the heater nip the end portions of the heater 108 and the heater holder 109 therebetween. The fixing film 111 is cylindrical heat-resistant film of three-layer structure. The innermost layer of the fixing film 111 is a base layer, i.e., a layer bearing mechanical characteristics such as the torsion strength and smoothness of the fixing film 111, and is formed of resin such as polyimide.
The next layer is an electrically conducting primer layer, i.e., an electrically conducting layer having electrically conductive particles such as carbon black dispersed therein. The electrically conducting primer layer serves also as an adhesive effecting the joint of the third layer and the base layer. The outermost layer is a top layer and is designed to have an optimum resistance value and an optimum film thickness so as not to cause various bad images. The reference numeral 110 designates a fixing stay formed of a metal such as iron or aluminum. The fixing stay 110 serves to suppress the deformation of the heater holder 109 by creeping and enhance the rigidity of the heater holder 109. The reference numeral 113 denotes flanges mounted on the opposite end portions of the fixing stay 110.
The heater 108, the heater holder 109 and the fixing film 111 fitted on the fixing stay 110 are located between the flanges 113 on the axially opposite end portions and are subjected to lengthwise regulation. The above-described assembly is a film unit.
The reference numeral 119 designates a pressure roller. The pressure roller 119 comprises a mandrel 119a made of aluminum or cast iron and covered with heat-resistant rubber 119b such as silicone rubber. The surface layer of the rubber 119b of the pressure roller 119 is provided with film of fluorine resin such as PFA, PTFE or FEP having a releasing property with respect to the toner. The pressure roller 119 has its axially opposite end portions rotatably journalled between the side plates of the heating apparatus, not shown. The aforedescribed film unit is opposed to the upper side of the pressure roller 119 so that the heater 108 may face downward, and the flanges 113 mounted on the fixing stay 110 are downwardly urged by pressure springs 116 to thereby form a fixing nip N.
The pressure roller mandrel 119a of the pressure roller 119 is rotatively driven by a pressure roller gear 115, and the fixing film 111 is driven to rotate in the fixing nip part N. The sheet P bearing the toner thereon is conveyed by the transferring roller 208 and the photosensitive drum 201 and is guided to the fixing nip part N by a fixing inlet guide 118. The toner T on the sheet P is pressed against the recording medium P and heated in the fixing nip part N, and the toner T is softened and closely adhereto to the sheet P and is permanently fixed. A heating member of low heat capacity can be used in the fixing apparatus of such a film heating type and therefore, as compared with the conventional heat roller type, the shortening of the waiting time (quick start) becomes possible. Also, by the quick start becoming possible, preliminary heating during the non-printing operation becomes unnecessary and overall saving of electric power can be achieved.
The prior-art heating apparatus, however, has suffered from the following problem.
In the conventional temperature detecting means 117, the protective sheet 103 has been of a shape completely covering the temperature detecting element 101 and the elastic member 102.
Accordingly, in order that the temperature detecting element 101 may reliably abut against the heater 108 with the protective sheet 103 interposed therebetween, it has been necessary to sufficiently secure a gap G3 between the protective sheet 103 and the heater holder 109, as shown in FIG. 8, to prevent the protective sheet 103 from interfering with and riding onto the hole 109a of the heater holder 109.
On the other hand, if the gap G3 becomes great, when the heater 108 generates heat, a temperature difference between a portion in which members (the heater holder 109 and the protective sheet 103) abutting against the upper surface side of the heater 108 are present and a portion (gap G3) in which they are absent, and an internal stress difference applied to the interior of the heater 108 becomes great, and this has caused the damage of the heater 108 in some cases.
Also, FIGS. 12A to 12D of the accompanying drawings show a fixing device having another conventional temperature detecting device mounted thereon, FIG. 12A being a plan view, FIG. 12B showing the free state of the temperature detecting device, FIG. 12C being a cross-sectional view taken along the line 12Cxe2x80x9412C of FIG. 12A, and FIG. 12D being a cross-sectional view taken along the line 12Dxe2x80x9412D of FIG. 12C.
In FIGS. 12A to 12D, the conventional temperature detecting device has a heat-resisting elastic member 2 provided with a temperature detecting element 1 on the underside thereof mounted on a temperature detecting element holding member 33 with a temperature detecting element holding surface 33a adjusted thereto, and the temperature detecting element holding member 33 is mounted on a positioning member 34 through two electrically insulated leaf springs 35a and 35b serving also as the lead wires of the temperature detecting element 1.
The positioning member 34 is formed with a slot-shaped positioning hole 34a and a circular positioning hole 34b. Also, harness 7 connected to the leaf springs 35a and 35b is drawn out of the positioning member 34, and is connected to a CPU.
The reference numeral 39 denotes a heating member holding member which is integrally formed with positioning projections 39a and 39b fitted in the positioning holes 34a and 34b of the positioning member 34. Also, the heating member holding member 39 is formed with a hole portion 39c so that the temperature detecting element 1 can contact with the ceramic substrate of the heating member 8 exposed in the hole portion 39c. 
The temperature detecting device in its natural state, as shown in FIG. 12B, is such that the leaf springs 35a and 35b are bent midway thereof and the temperature detecting element holding member 33 is in its downwardly facing posture, and is designed such that by the positioning member 34 being mounted on the heating member holding member 39, the pressure of the surface of contact between the temperature detecting element 1 and the heating member 8 is applied thereto by the resilient deformation of the leaf springs 35a and 35b. 
Also, the positioning member 34 is designed such that the radial position of the positioning member 34 is determined by the fitting between the positioning holes 34a, 34b and the projections 9a, 9b, and the thrust direction of the positioning member 34 is fixed and held by a fixing member, not shown.
As shown in FIGS. 12A to 12D, the temperature detecting device is positioned relative to the heating member holding member 39 and the heating member 8 by the positioning member 34, and is connected to the temperature detecting element holding member 33 with the leaf springs 35a and 35b, and is designed such that the contact pressure between the temperature detecting element and the heating member is ensured by the action of the bending stress of the leaf springs.
FIG. 13 of the accompanying drawings schematically shows the relation between the contact pressure and the detected temperature, and the axis of abscissas is indicative of the contact pressure and the axis of ordinates is indicative of the output of the temperature detecting element, and the shown graph graphically shows changes in the output when the contact pressure has been changed when the temperature is constant.
As shown, when the contact pressure is changed, the result of the detection has a characteristic of changing and therefore, actually, the shown range in which the gradient is small is a utilizable range, but the gradient is never 0, but it is an important design task leading to accurate temperature detection, and further to the higher speed of response and the optimization of temperature control to more stabilize the contact pressure.
The present invention has been made in view of the above-noted problems and an object thereof is to provide an image heating apparatus in which the abutting state of a temperature detecting element against a heater is optimum.
Another object of the present invention is to provide an image heating apparatus which is excellent in the accuracy of temperature detection.
Still another object of the present invention is to provide an image heating apparatus comprising:
a heater; and
temperature detecting means for detecting the temperature of the heater, the temperature detecting means having a temperature detecting element, an elastic member holding the element and film covering the elastic member;
the width of the film being smaller than the width of the elastic member.
Yet still another object of the present invention is to provide an image heating apparatus comprising:
a heater;
a holder for holding the heater; and
temperature detecting means for detecting the temperature of the heater, the temperature detecting means having a temperature detecting element and a supporting member for holding the element;
wherein the temperature detecting element detects the temperature of the heater through a hole formed in the holder, and the holder has positioning portions for positioning the supporting member at the right and left of the hole of the holder.
Further objects of the present invention will become apparent from the following detailed description when read with reference to the accompanying drawings.